Daniel Reetz, the founder of the DIY Book Scanner community, has recently started making videos of prototyping and shop tips. If you are tinkering with a book scanner (or any other project) in your home shop, these tips will come in handy. https://www.youtube.com/channel/UCn0gq8 ... g_8K1nfInQ

I've been looking at these for smartphone scanning as well to avoid using digital zoom. However, results are acceptable even from an iPhone 4s (see samples I've posted here: viewtopic.php?f=14&t=3401) with upto 2x digital zoom (depending on what you're trying to accomplish).

The zoom smartphone lenses are pretty useless from what I've seen. Image quality suffers greatly as you move away from the centre and, given we want to scan text, image quality is essential (even if we assume we'll lose 15% of the field of view).

Telephoto is probably what you need (you can tell exactly if you put your phone on the scanner and see if you need to zoom in to get the page to fill the image). That said, prices for a good lens range all over the map, from $20 to $200.

Here are reviews of the 3 best I've found (I'll be getting the Aukey telephoto myself -- based on the excellent test chart image at the review below):